Method of removing co2 and h2o from a gas stream



July-7, 1970 R. ENGEL ETAL 3,519,384

METHQD OF REMOVING CO AND H O FROM A GAS STREAM Filed Aug. 15. 1966United States Patent 3,519,384 METHGD 0F REMOVING C0 AND H 0 FROM A GASSTREAM Rudolf Engel, Aachen, and Claus-Benedikt von der Declren,Verlautenheide, Aachen, Germany, assignors to Brown Boveri/ KruppReaktorbau G.m.b.H., Dusseldorf, Germany Filed Aug. 15, 1966, Ser. No.572,477 Claims priority, application Germany, Aug. 19, 1965, B 833,293Int. Cl. B0111 53/04 US. Cl. 23-2 6 Claims ABSTRACT OF THE DISCLOSURE Amethod of removing CO and H 0 from a gas stream in which threeabsorption filter elements are used. The first filter element initiallycontains Ca(OH) and the second and third filter elements initially CaO.The method comprises initially passing the gas stream sequentiallythrough the first and second filter elements which operate respectivelyas a C0 absorption filter and as an H O absorption filter, convertingthe Ca(OH) and the CaO respectively to CaOO and Ca(OH) The thirdabsorption filter element is kept in reserve until the first filterelement is exhausted; at this time the gas is cycled sequentiallythrough the second filter element, which now contains Ca(OH) and thethird filter element. The first filter element may now be charged withCaO and held in reserve to act as an H O absorption filter when thesecond filter element is exhausted.

This invention relates to a method of removing C0 and H 0 from a gasstream.

The present invention is concerned with such a method in which lime isused as an absorbent in several mutually disconnectable absorbent filterelements which can be selectably interposed in the gas stream.

Many processes performed in industrial plant involve the formation ofmore or less considerable volumes of CO which adversely affect thefurther course of the process, and which are not therefore desirable.For instance, in a nuclear reactor the reaction of graphite with oxygenor of graphite with water continuously leads to the formation of CO andCO which then contaminates the gaseous coolant circulating through thereactor. By oxidation, the CO can be readily converted to CO and thelatter can be removed from the coolant system by a reaction with limeaccording to the equation The water produced in this reaction islikewise undesirable and must be removed in another appropriatereaction.

Moreover, in some types of nuclear reactor, the gaseous coolant may beadditionally contaminated with water introduced into the gas by thesteam generator or by the carbon of the reflector. This water mustsimilarly be removed from the circulating gas.

Apparatus is already known in which two absorption filters are providedworking in parallel and containing a packing of Ca(OH) to remove fromthe gas stream that is to be purified the entrained CO by converting thesame into CaCO The water that is simultaneously formed is absorbed in asubsequent H O absorption filter which is packed with CaO. Only one ofthe two CO absorption filters is introduced into the gas stream at atime whilst the other filter is reactivated by an abrupt increase intemperature followed by cooling. The introduction of the filters intothe gas stream and their removal therefrom is controlled by a three-wayvalve which is electrically controlled. Apparatus of such a kind con-3,519,384 Patented July 7, 1970 verts a considerable quantity of theCa(OH) used into CaCO and the cleaning eflfect is high. However, theapparatus has two major drawbacks. First, the heating elements requiredfor raising the temperature of the exhausted filters are highly stressedby the continuous heating and cooling, so that their life is short.Second, the use of an electrically controlled three-way valve isdifficult in a large-scale industrial plant.

It is therefore the object of the present invention to provide a methodof removing the CO and H 0 from a stream of gas, which is just aseffective and permits the same cleaning effect to be achieved as inconventional apparatus, but which at the same time avoids theabovementioned drawbacks. Moreover, it is desirable even further toimprove the percentage utilisation of the absorption filters. With thisend in view the present invention consists in providing three absorptionfilter elements whereof the first initially contains a packing ofgranular Ca(OH) and the second and third filter elements contain apacking of CaO, the method comprising initially passing the gas streamsequentially through the first and second filter elements and operatingthese respectively as a C0 absorption filter in which the Ca(OH) isconverted into CaCO and as an H O absorption filter and during suchoperation keeping the third filter element disconnected and in reserve,the method further comprising disconnect ing the gas stream from thefirst filter element when this is exhausted and then connecting thesecond and third filter elements so that the gas stream passessequentially through them with the result that the second filter nowacts as a C0 absorption filter and the third filter element acts as an HO absorption filter and, during this operation, the first filter elementis re-charged with a fresh CaO packing and held in reserve to act as anH O absorption filter when the second filter element is exhausted andreplaced as a C0 absorption filter by the third filter element.

In the method according to the invention a three-way valve in the gasentry duct can be dispensed with and a sudden surge-like increase intemperature followed by cooling for the purpose of reactivating the CO-filter is not needed, so that the drawbacks this involves in knownarrangements do not arise.

The Co -absorbing packing consists of an extremely reactive Ca(OI-l) andthe CO absorption filter is initially operated at a working temperatureof C. which is raised by 10 to 20 C. increments in the course of theprocess. The reason for this progressive temperature rise is that afterthe conversion of about 2.9% of the Ca(OI-l) to CaCO a thin film of CaCOwill have formed which will envelop the grain of the absorbent packingand thereby prevent a continued reaction of the CO with the Ca(OH) Atthis stage the filter is therefore for the time being saturated.However, if the temperature of the CO -filter is now raised by about 15C., and maintained until saturation point is reached again, the rise invapour pressure associated with the increase in temperature issulficient to re-open the pores inside the packing grains which had beenchoked by the CaCO that had formed. Consequently fresh Ca(OH) is againexposed. When more of the =Ca(OH) has reacted and the pores of thegrains have again become choked with CaCO a fresh temperature riseaccompanied by an increase in vapour pressure again exposes fresh Ca(OH)and the 'cO -filter continues to function satisfactorily as before.These temperature increments can be continued until a temperature ofabout 400 C. has been reached, at which stage about 70% of the Ca(OH)will have reacted.

The H O-filter which follows the Co -filter must contain an absorbentwhich even at the elevated temperatures will not release the absorbedwater. Moreover, the reaction product of the absorbent with H O must notbe a gas, a liquid or an aggressive chemical compound, as in the case inthe majority of known chemical drying agents. The best packing materialhas been found to be a CaO which has not been produced in the manner theusual commercial grade of CaO is produced, namely from CaCO by calciningthe same, but from Ca(OH) by the following reaction:

The CaO that is obtained in this way is far superior to a CaO obtainedfrom CaCO in so far as its reactivity with water is concerned.

The absorbent in the H O-filter reacts with the water that is to beremoved from the gas according to the reaction Up to 80% of the CaO inthe H O-filter is thus converted to Ca(OH) Advantage can be taken ofthis reaction scheme in the proposed method by re-using exhausted HO-filters as Co -filters and thus utilising the absorption filterstwice. To this end three filters are provided for inter-position in thecirculated gas stream of which two contain a CaO-packing when theprocess begins, whereas the third contains Ca(OH) grains. One of thefilters containing a CaO-charge is first kept in reserve, whereas theother is connected into the gas stream as an H O-filter on thedownstream side of the CO filter. The Co -filter extracts the CO fromthe gas that is to be cleaned but releases water, the charge of Ca(OH)contained in this filter being converted in stages to CaCO as thetemperature is raised in consecutive increments up to 400 C. Meanwhilethe CaO in the H filter has been converted to Ca(OH) by taking the waterout of the gas, so that when the CO breaks through the CO -filter the HO-filter can undertake the functions of the Co -filter. The exhaustedfilter is taken out of the circulating gas stream, provided with a freshcharge of CaO and held in reserve. Meanwhile the filter which had beenin reserve is operated as the 'H O-filter. When the absorption filternow operating as a CO -filter breaks through upon reaching a temperatureof 400 C., the next H O-filter takes its place and the filter which isnow in reserve is introduced into the gas stream as an H O- filter. Itwill thus be understood that each filter is used twice.

In the method proposed by the invention the working temperature of theCo -filter is conveniently raised in stages under the control of a CO-meter connected to the gas duct on the downstream side of theabsorption filter functioning as the Co -filter. The CO -metercontinuously measures the concentration of CO on the downstream side ofthe CO -filter and activates a temperatureregulating device as soon asthe filter reaches the point of saturation, said temperature regulatorraising the temperature of the filter in the above described manner. Formonitoring the H O-concentration in the gas stream on the downstreamside of the H O-filter a hygrometer may be built into the gas duct.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawing which illustratesdiagrammatically filtration apparatus for carrying out the invention.

Referring now to the drawing there is shown a first absorption filter 1,a second absorption filter 2 and a third absorption filter 3. -All threefilters 1, 2 and 3 are connected in parallel both at their inlet andoutlet ends to the flow and return parts respectively of a gascirculating pipe 4.

The first filter 1 contains a packing of Ca (OH) grains. An inlet stopvalve 5 is provided at the filter inlet and an outlet valve 6 at thefilter outlet. Upstream of the outlet stop valve 6, the first filteroutlet is connected by a conduit 7 to the inlet of the second filter 2.Downstream of the inlet valve 5, the filter inlet is connected by aconduit 8 to the outlet of the third filter 3. Stop valves 9 and 10 areprovided in conduits 7 and 8 respectively. Thus it will be seen that (i)by closing stop valves 9 and 10 and opening valves 5 and 6, the filter 1will be connected to the gas circulating pipe 4, (ii) by closing valve 6and opening valve 9, the first filter outlet will be connected to thesecond filter inlet, the first filter inlet remaining connected to thegas pipe 4, (iii) by closing valve 5 and opening valve 10 and by openingvalve 6 and closing valve 9, the first filter inlet will be connected tothe third filter outlet and first filter outlet will be connected to thegas pipe 4, and (iv) by closing valves 5, 6, 9 and 10 the first filter 1may be isolated.

The second filter 2 is filled with a packing of CaO which has beenproduced from Ca(OH) and which operates as an H O absorption filter. Aninlet stop valve 11 is provided between the second filter inlet and thegas pipe 4. An outlet stop valve 12 is provided between the secondfilter outlet and the gas pipe 4. Upstream of the outlet stop valve 12,the second filter outlet is connected by a conduit 13 to the thirdfilter inlet. A stop valve 14 is provided in the conduit 13. In asimilar manner to that described in the preceding sentence in relationto the first filter 1, the second filter 2 can be connected (i) at itsinlet and outlet to the gas circulating pipe 4, (ii) at its outlet tothe third filter inlet and at its inlet to the gas circulating pipe 4,(iii) at its outlet to the gas circulating pipe 4 and at its outlet tothe first filter outlet pipe, and (iv) it may be isolated.

The third filter 3 also contains a packing of CaO which has beenproduced from Ca(OH) An inlet stop valve 15 is provided between thethird filter inlet and the gas pipe 4. An outlet stop valve 16 isprovided between the third filter outlet and the gas pipe 4. In asimilar manner to that described in relation to the first filter 1, thethird filter 3 can be connected (i) at its inlet and outlet to the gascirculating pipe 4, (ii) at its outlet to the first filter inlet and atits inlet to the gas circulating pipe 4, (iii) at its outlet to the gascirculating pipe 4 and at its inlet to the second filter outlet and (iv)it may be isolated.

A stop valve 17 is provided in the gas circulating pipe 4 between theflow and return parts thereof, i.e., between the inlet connections andthe outlet connections to the three absorption filters 1, 2 and 3.

Each of the three absorption filters 1, 2 and 3 is provided with aheating element 18. The heating elements 18 are separately controllableby a temperature regulating unit 19. A Co -meter 20 is connected by aconnecting line 21 to the temperature regulating unit 19 to control thisunit. The three filter outlets are connected, upstream of the outletvalves to the CO -meter 20. Stop valves 23, 24 and 25 are provided todisconnect the first, second and third filters respectively from acollecting pipe 22 leading to the CO -meter 20. An isolating stop valve26 isolates the CO -meter from the collecting pipe 22 and from all thefilters. The return end of the gas circulating pipe 4 is finallyconnected by a short branch pipe controlled by a valve 28 to ahygrometer 27.

To start with, all the valves are so adjusted that the third filter 18is isolated and the gas that is to be purified flows from the flow partof the pipe 4 through the first absorption filter 1, and from the firstfilter outlet to the second filter inlet and thence to the return partof the pipe 4. The CO is retained in the first absorption filter 1 andthe water in the second absorption filter 2. The CO concentration in thegas leaving the first filter 1 is continuously monitored by the Co-meter 20 which automatically activates the temperature regulator 19when the first filter 1 reaches saturation point. The startingtemperature of the first filter 1 is C. and is raised in stages, from180 C., in increments of about 15, to a final temperature of 400 C. whenthe CO finally breaks through the first filter 1.

The valves are then reset in such a way that the first absorption filter1 is isolated and the gas is instead first admitted from the flow partof the pipe 4 into the inlet of the second absorption filter 2, from thesecond filter outlet into the inlet of the third absorption filter 3.The second and third absorption filters new function as CO and H 0absorption filters respectively. Meanwhile the exhausted first filter 1is charged with a fresh CaO packmg.

When the second absorption filter 2 is finally exhausted at atemperature of 400 C., the third absorption filter 3 takes its place andfunctions as the CO -filter, whereas the first filter 1 is brought intooperation as an H O-filter. Hence each filter consecutively performs twofunctions, the reaction product in the H O-filter being used asabsorbent when this filter is taken into use as the CO -filter.

We claim:

1. A method of removing CO and H 0 from a gas stream using threeabsorption filter elements in which a first filter element initiallycontains a packing of Ca(OH) and a second and a third filter elementsinitially contain a packing of CaO comprising:

(A) initially passing the gas stream sequentially through said first andsaid second filter elements, and

(1) operating said first filter element as a C0 absorption filterwhereby heated Ca(OH) acts to absorb CO and is thereby converted to CaCO(2) operating said second filter element as an H O absorption filterwhereby CaO acts to absorb H 0 and is thereby converted to Ca(OH) and(3) maintaining said third filter element in reserve;

(B) disconnecting said gas stream from said first filter element when itis exhausted as a C0 absorber; and

then

(C) passing said gas stream sequentially through said second and saidthird filter elements, and

(1) operating said second filter element, which now contains Ca(OH) as aC0 absorption filter whereby heated Ca(OH) acts to absorb CO and isthereby converted to CaCO (2) operating said third filter element as anH O absorption filter whereby CaO acts to absorb H 0 and is therebyconverted to Ca(OH), and

(3) charging said first filter element with a CaO packing andmaintaining it in reserve whereby it may subsequently be used as an H Oabsorption filter and, after said second filter element is exhausted asa C0 absorber, employed in conjunction with said third filter elementoperated as a C0 absorption filter.

2. A method according to claim 1 further comprising raising thetemperature of the filter acting as a C0 filter above its startingvalue, as it becomes increasingly exhausted to raise the absorptioncapacity thereof.

3. A method according to claim 2 wherein the temperature of the saidfilter acting as a C0 filter is raised in a plurality of stages.

4. A method as claimed in claim 3 wherein the temperature is raised fromC. to 400 C. in stages of about 10 to 20 C.

5. The method of claim 1 in which said filter element which is acting asa C0 filter is operated at a temperature of at least 180 C.

6. The method of claim 3 comprising connecting the outlet side of thefilter element which is acting as a C0 filter to a C0 meter whichoperates to measure the CO content of the gas stream at the outlet sideof the CO filter, said CO meter operating through a temperatureregulator whereby the temperature of said CO filter is increased when apredetermined amount of CO is present in said gas stream at the outletside of said CO filter.

References Cited UNITED STATES PATENTS 10/1943 Biederbeck et al. 2328/1963 Dulfey 232 X US. Cl. X.R. 23-66

